In-place reserves of heavy oil in the United States have been estimated about one hundred fifty billion barrels. Of this large in-place deposit total, however, only about five billion barrels may be considered economically produceable at current oil prices. One major impediment to production of oil from such deposits is the high viscosity of the oil. The high viscosity reduces the rate of flow through the deposit, particularly in the vicinity of the well bore, and consequently increases the capital costs per barrel so that overall costs per barrel become excessive.
Various techniques have been tried to stimulate flow from wells in heavy oil deposits. One technique utilizes steam to heat the oil around the well; this method has been utilized mostly in California. However, steam has drawbacks in that it is not applicable to thin reservoirs, is not suitable for many deposits which have a high clay content, is not readily applicable to off-shore deposits, and cannot be used where there is no adequate water supply.
There have also been a number of proposals for the use of electromagnetic energy, usually at conventional power frequencies (50/60 Hz) but sometimes in the radio frequency range, for heating oil deposits in the vicinity of a well bore. In field tests, it has been demonstrated that electromagnetic energy can thus be used for local heating of the oil, reducing its viscosity and increasing the flow rate. A viscosity reduction for oil in the immediate vicinity of the well bore changes the pressure distribution in the deposit to an extent such that flow rates may be enhanced as much as three to six times.
Perhaps the most direct and least costly method of implementation of electromagnetic heating of deposits in the vicinity of a well bore utilizes existing oil well equipment and takes advantage of conventional oil field practices. Thus, conventional steel well casing or production tubing is often employed as a part of the conductor system which delivers power to a main heating electrode located downhole in the well, at the level of the oil or gas deposit. However, the high magnetic permeability of a steel casing or tubing, with the associated eddy current and hysteresis losses, often creates excessive power losses in the transmission of electrical energy down through the wellbore to the main electrode. Such power losses are significant even at the conventional 50/60 Hz supply frequencies that are used almost universally. These losses may be mitigated by reducing the A.C. power frequency, as transmitted to the downhole heating electrode, but this creates some substantial technical problems as regards the electrical power source, particularly if the system must be energized from an ordinary 50/60 Hz power line.
Many of the technical difficulties in the use of low frequency A.C. power in heating oil and like deposits to improve well production are effectively solved by the power sources described and claimed in the co-pending, U.S. patent application of J. E. Bridges et al Ser. No. 322,012 filed simultaneously herewith, now U.S. Pat. No. 4,919,201. But other problems, particularly corrosion problems, remain.
A major difficulty with the use of low frequency A.C. power for localized heating of deposits in a heavy oil well arises because corrosion effects at low frequencies (e.g., below thirty-five Hz) are substantially enhanced in comparison with the corrosion that occurs in heating systems using conventional power frequencies of 50/60 Hz. Thus, for extended well life it is important to incorporate cost effective corrosion protection in the heating system.
Conventional corrosion protection arrangements for pipelines and oil wells usually include coating the pipe, casing, tubing, etc., of whatever configuration, with a layer of insulator material. In an electromagnetic heating system for an oil well, which must deliver power to a main heating electrode located far downhole at the oil deposit level, a secondary or return electrode is also required. That is, there are two exposed, uninsulated electrodes in the system, a main electrode downhole in the region of the oil deposit and a return electrode spaced from the main electrode. The secondary electrode is usually located above the deposit. To maintain conduction and heating, these electrodes must be positioned so that electrical energy flowing between them passes through a localized portion of the deposit. Accordingly, surface insulation can be used on only a portion of the electromagnetic well heating system. The most critical element, of course, is the exposed main heating electrode located downhole in the deposit; it cannot easily be replaced. Thus, corrosion damage to the downhole main heating electrode may shorten the life of the heating system substantially and may greatly reduce its economic value.
Further, maintaining the electrode in the deposit at too large a negative potential can cause a buildup of scale that may plug casing perforations or screens in this part of the well. Such excess scale accumulation at the downhole electrode is quite undesirable. Depending on the specifics of the application, it may be desirable to reduce the D.C. component of the current between the electrodes to as small a value as possible or to hold the downhole electrode at the least practical negative potential. This suppresses scale buildup on the reservoir electrode and reduces anodic corrosion losses at the return electrode.
Cathodic protection has been widely used for pipelines, oil wells, and other similar applications. This technique involves maintenance of a buried metal component, insulated or exposed, at a negative potential with respect to the earth. In this way, positive metallic ions that would normally be driven out from the buried metal element are attracted back into it, suppressing the corrosion rate. Of course, this requires that another exposed metal element or electrode be placed in the earth and maintained at a positive potential. In cathodic protection, as otherwise in the physical world, there is no free lunch. The positive D.C. potential of the secondary electrode drives the positively charged metallic ions into the earth and causes corrosion at the secondary electrode, the anode, at a rate that is a function of the D.C. bias current and the metallic constituents of the anode. Consequently, the positively charged return electrode is sometimes called the "sacrificial electrode". Sacrificial electrodes are usually designed either to be replaced or to have sufficient metal or chemical constituents so that they can withstand continued corrosion losses over an acceptable life for the system. Long life secondary electrodes (e.g., high silicon steel) are of material assistance in keeping secondary electrodes in service, but even this expedient is inadequate if large D.C. currents are tolerated.
Conventional cathodic protection systems cannot . handle the large A.C. currents (e.g., 50 to 1000 amperes) often required for effective electromagnetic downhole heating in oil wells and like mineral fluid wells. This is especially true for currents in a low frequency range, such as between 0.01 and 35 Hz. Another difficulty with some of the known cathodic protection systems is that they are predicated upon application of a fixed potential large enough to assure that the protected metallic equipment (in this instance the downhole main heating electrode) is always negative with respect to the earth. But corrosion related currents and voltages vary with changes in heating currents. For an electromagnetically heated oil well, the rate of heating required for efficient operation may vary with changes in the production rate of the well, its oil/water ratio, the electrochemical constituents of the reservoir fluids, and other factors. Even in non-reservoir formations, these phenomena impose variable requirements with respect to the D.C. corrosion-protection bias. As a consequence, for most conventional cathodic protection systems excessive voltage requirements are imposed, with the result that there is excessive corrosion (and loss of efficiency) at the return electrode. The return electrode is likely to be over-designed and undesirably expensive; D.C. power requirements are also excessive.
There is another type of oil well heating system in which the heat is applied to the flow of oil within the well itself, rather than to a localized portion of the deposit around the well. Such a heating system, usually applied to paraffin prone wells, is described in Bridges et al U.S. Pat. No. 4,790,375, issued Dec. 13, 1988. In a system of this kind the heating element or elements constitute the casing, the production tubing, or both; the high hysteresis and eddy current losses in steel tubing make its use frequently advantageous. In such systems it is frequently desirable to supply heating power to the system at frequencies substantially above the normal power range of 50/60 Hz, but corrosion problems generally similar to those in low frequency deposit heating systems may occur.